It’s finally worked, although I’m afraid I’m not really sure why, so not much help to anyone else having the same problem.
Here’s some of the things I did:
1. Moved all my Final Cut files to my main hard drive (they were on an external one), which I figured should speed things up.
2. Exported a QT movie
3. Dragged the QT movie into my timeline and exported using QT conversion. (QT conversion wouldn’t work for me straight from timeline).
4. Used compressor the settings in the post above.
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